Your research plan will have 3 sections in which you will answer the following questions. It must cite at least 5 peer reviewed articles in APA 7th format.
PsycINFO: Core database in psychology, from the APA. Great search feature that lets you find empirical research on a topic.
PubMED: Powerful database with research from not just medicine, but also neuroscience, psychology, cognitive science and more.
MIT CogNET: Digital library covering the brain and cognitive sciences. Includes reference works, journals, books and more.
ScienceDirect: Multidisciplinary collection of journals and ebooks with an emphasis on the sciences.
When there are only a few articles on a specific topic, two strategies can help -- looking at the references cited by the articles you did find, and seeing who has cited them. Some databases have a built in "cited by" feature, Google Scholar is also very helpful in locating additional citations.